The present invention relates generally to secrecy communication systems and more particularly to systems for narrowband analog transmission of messages with privacy.
It is a principal object of the present invention to provide a speech privacy system in which an analog message signal is linearly mixed with a noiselike signal to produce an apparently random composite waveform wherein the original message signal is completely masked and is unavailable to all except authorized users of the system.
A wide variety of communication secrecy or privacy systems have been proposed in the past twenty-five years, most of these characterized in that one or more parameters of the message are varied or modified in some arbitrary or random fashion at the transmitter, and these scrambled or jumbled parameters subsequently returned to their original form at the receiver of an authorized party. In general, decoding is accomplished from a knowledge of the scrambling technique used at the transmitter and by means of some form of synchronization of transmitted and received signals and their scrambling and unscrambling waveforms.
Among the first techniques of masking intelligible messages was the addition of "noise" to the message to produce a signal buried in obscuring noise. At the receiver, the output of a local source of noise corresponding identically to that used at the transmitting terminal, and synchronized with the transmitter noise source and the message signal, was subtracted from the signal plus noise to produce the original message. Obviously, such a system is relatively ineffective to prevent "evesdropping" because it is merely necessary to suppress the noise in some suitable fashion whereby to obtain the recognizable signal pattern buried therein.
A subsequent system was suggested in which signal parameters such as amplitude and phase were altered according to frequency, prior to adding noise thereto, in order to provide an additional quantity unknown to those unauthorized persons seeking to unscramble the message. It was then much more difficult to obtain the desired information, absent identical synchronized demodulation equipment at the receiver, since there was involved more than a simple subtraction or suppression of noise from signal. Nevertheless, the additional pattern by which the signal was modified was a regular, (i.e., not random) format and could conceivably be derived in short order by suitable iterative or trial and error techniques. This was particularly true because once the noise was suppressed, at least some regular format was observable, and it then remained only to find the key by which that format was modified on the basis of frequency or phase, or both.
Another approach previously taken in speech privacy transmission systems involves the provision of means for scrambling message waves in an arbitrary manner approaching a random or a pseudo-random order extending over a lengthy period, before a repetition of the complete code cycle is begun to transmit further message fragments. Privacy is enhanced since an unauthorized party must first discover the code element by element because of the lack of a recurring scheme of scramble within the long code cycle by which to enable decoding of the message in blocks of substantial length.
Still another prior art system utilizes the diverging of the frequency order of the speech or signal wave by modulation of a continuous wave of appropriate frequency, and selection of the lower sideband for transmission. Additional irregularity is introduced by inserting non-cyclic variations into the inverting wave itself, these variations being non-repeated during the message transmission.
In yet another secrecy communication system the speech amplitudes are first converted into pulse combinations and are subsequently enciphered by employment of telegraphy coding methods. The pulse combinations are obtained by a form of speech quantizing together with scanning to produce a code in which each pulse combination corresponds to a speech amplitude lying between two specified limits, and the variable amplitude of the speech is then transmitted in the form of a sequence of these pulse combinations.
Still another method of operating a secrecy communication system involves alteration of an intelligence or message signal by abruptly varying a characteristic of the signal at predetermined intervals in accordance with a coding schedule. The altered intelligence signal is then sampled at points in time differing from the times at which the aforementioned characteristic was abruptly varied to produce an output signal consisting of the sampled portions of the altered intelligence signal. Before transmission, the output signal wave form is shaped to simulate that of the altered intelligence signal prior to sampling.
According to still another approach to secrecy communication, a series of signal generators individually producing a signal having an indentifiable characteristic are actuated in a random sequence. One of the signal generators is also randomly selected for actuation in accordance with operating condition of the mechanism for random selection of the overall series of signal generators so as to produce a series of code bursts.
In another security communication system the coding is effected on the basis of a plurality of mutually orthogonal functions resembling noise in appearance, and the message signal sampled in accordance with this coding technique is transmitted by means of code groups representing amplitude of samples which are substantially randomly distributed by means of the same sampling technique to several different carrier channels.
A still further method and apparatus for masking communication signals in the prior art has consisted of generating at the transmitting terminal of the system a sequence of pulses one parameter of which is modulated by a combined signal consisting of communication signal and a concealing supplementary signal by use of a sawtooth switching arrangement. A similar switching arrangement is utilized to decode the pulses picked up at the receiver. In still another method for camouflaging communication signals a first series of pulses modulated by the intelligence signal is combined at the transmitting station with an additional series of pulses of arbitrarily varying polarity, to produce a composite pulse series. A series of control pulses is transmitted along with the composite pulse series to the receiving station where an arrangement identical to that used at the transmitting station is employed to reproduce the aforementioned additional series of pulses for application to the composite pulse series, to reconvert the latter into the original series of pulses.
According to another speech security system of the prior art, a low amplitude quieting voltage having a frequency at the lower end of the system passband is applied to a speech signal into which randomly timed phase reversals have been introduced, by which to enhance the scrambling of the transmitted signal. A special squelch circuit is utilized to suppress any audio output of the system in the absence of speech so as to eliminate the otherwise noticeable intersyllable noise.
In another prior art privacy system the intelligence signal is scrambled by passing it through a linear filter at a transmitting station whereby to add time inverted reverberation to the signal and thus provide it with a substantial number of pre-echos of amplitude and polarity which render it unintelligible to unauthorized receivers.
While such prior art methods, both digital and analog, have served some utility as message privacy systems, each has required synchronization between transmitter and receiver and each is further generally characterized by system complexity of an extent which has thus far rendered privacy systems to be of prohibitive cost.
It is therefore a further object of the present invention to provide a speech privacy system capable of narrowband transmission of analog signal in an unrecognizable composite waveform, in a manner that overcomes one or more of the disadvantages of the prior art privacy or secrecy systems.